For many applications, relays, especially old-fashioned electromechanical relays, are the simplest and most reliable way to switch currents to load. This is particularly true for situations where the load currents may be high, e.g., in excess of 20 amperes.
However, the cost, size and weight of electromechanical relays increases significantly as their current carrying capacity goes up. In many circuit configurations where cost and/or size and/or weight is a consideration, it would be desirous to replace a single large high current capacity relay with two or more smaller relays in parallel, each of which has a current capacity below that required by the circuit application, but whose combination provides the desired capacity.
While this at first appears to be a simple matter, the use of parallel relays to increase the overall load current capacity of a circuit is not recommended by relay manufacturers, because such parallel relay configurations often lead to premature relay failure.
The key failure mechanism that occurs when two or more relays are connected in parallel arises from time differences that occur between the opening and closing of the contacts in such parallel-configured relay assemblies. When one attempts to operate two or more relays in parallel in place of a single larger relay, inherent manufacturing tolerances will invariably cause one of the relays to open or close at a slightly different time as compared to the other relay(s).
As a consequence, when the relays in a parallel relay assembly are switched to close, the relay that closes first will momentarily have to pass a load current that is significantly above its rated current carrying capacity, until the other relay (or relays) close. Likewise, when the relay assembly is switched open, the relay that opens last will also exceed its current carrying capacity, after the other relays have dropped out. Repeated overcurrent of the relay contacts during these transient switching time periods ultimately leads to their premature failure.
In accordance with the embodiments taught herein, such failure mechanism is avoided by incorporating one or more semiconductor devices that commutate the current through the arrangement of parallel relays during the times when their contacts or opening and closing, to avoid over current and attendant arcing from damaging the contacts.
By arranging for the bulk of the switching current to pass through one or more semiconductor devices during turn-on or turn-off of the parallel relays, rather than through their relay contacts, the contacts will carry an insignificant current during the switching operation. This eliminates the possibility of a single relay carrying more than it's rated current at any time.
By way of further background, when using electromechanical relays, care must also be taken to avoid or reduce problems arising from contact bounce, arcing and other deleterious transient effects that often occur during the making and breaking of the relay's mechanical contacts. Such effects are particularly problematic when switching highly inductive loads. These transient effects, if not properly addressed, will result in contact pitting, erosion, welding and ultimately in relay failure.
To reduce such problems caused by switching transients, it has been known in the art to incorporate, for example, an RC snubber circuit or a solid state switch in parallel with an electromechanical relay. Such snubber/switch arrangements generally may act to suppress destructive transients and prolong the life of the electromechanical contacts by providing an alternative path for current to flow during the make and break of the relay's contacts.
Prior art examples showing the parallel combination of an electromechanical relay with a solid state switch may be found in, for example, U.S. Pat. Nos. 3,639,808, 5,699,218, 8,482,885, among others.
However, the prior art only teaches use of a solid state switch to prolong the life of a single mechanical relay. Significantly, it has heretofore not been recognized that the use of solid state switches can also overcome the aforementioned problems inherent in using parallel combinations of electromechanical relays in place of a single larger electromechanical relay. As a consequence, it is still standard practice to avoid combining multiple relays in parallel, because of the problems discussed above.
Accordingly, a first advantage of the disclosed embodiments is to enable use of multiple smaller relays in parallel to switch currents that would otherwise require use of a larger, bulkier and more expensive single relay.
The embodiments disclosed herein also substantially eliminate arcing between contacts during the switching process, thereby causing little or no contact degradation during the operation of the relays. This results in extended relay life.
Further, since the semiconductor device only carries current for a short time corresponding to the transient time period during which the relay contacts are being switch from their fully closed to fully open states, a relatively small and inexpensive semiconductor device may be used.
Still further, since the parallel relays carry all of the current during normal operation, the semiconductor device will dissipate virtually no heat, obviating the need for large heat sinks.
Finally, a further advantage of the systems and methods disclosed herein is that arcing caused by mechanical vibration of the relay contacts, which may also result in premature failure, is also substantially eliminated.